Transducers comprise a family of sensors which are responsive to an input for providing a different type or term of output. For example, pressure sensor transducers are responsive to an input fluid pressure to deflect a diaphragm and provide an output change in capacitance. Sophisticated circuits are utilized to very accurately transform the small changes in capacitance into corresponding electrical signals which are representative of the change in input pressure. While it would be highly desirable to sense the amount of diaphragm deflection directly, such small changes in miniature transducers are often at the atomic level and thus not measurable with practical equipment. As a result, it is more practical to measure small changes in capacitance of the transducer and correlate the result thereof to a change in diaphragm deflection, and thus to changes in pressure, acceleration, etc., to which the transducer is responsive.
Deflections of clamped diaphragms in capacitive displacement type transducers, in relation to fixed plates produce corresponding changes in the capacitance of the members. In practice, such a structure includes two capacitors, one (C1) existing between the first plate and the diaphragm, and the second capacitor (C2) existing between the second plate and the diaphragm. As the diaphragm moves toward one plate or the other in response to pressure changes, the capacitances between the diaphragm and the two plates change accordingly, albeit in a nonlinear manner. Therefore, by measuring the change in the capacitance between the diaphragm and the plates, the amount of deflection can be measured.
Transducer circuits currently available attempt to directly measure the difference in the capacitances (C1-C2) of the transducer and produce an output which is proportional to the difference. However, the deflection of the transducer diaphragm is generally not linear with respect to C1-C2, and therefore complicated and expensive methods and devices are often required to both correct the nonlinearities and to compensate for other transducer or circuit inaccuracies.
Further, as transducer devices become miniaturized, the stray capacitance becomes very significant, often being larger than the capacitance of the transducer capacitors C1 and C2 being measured. This can be appreciated in view that with miniature transducers, the total capacitance may be in the neighborhood of several picofarads, with changes in the capacitance due to pressure changes being a matter of femtofarads.
Further still, traditional circuit designs require the use of precision and costly components. This can be realized as the differences of a few percent in the values of the resistors, capacitors, and other components can introduce unacceptable errors in the conversion of transducer capacitance to electrical output signals. Also, inaccuracies can occur due to thermal effects on the transducer and from changes in the dielectric constant between the plates and the diaphragm due to humidity, smoke, or other fluids passing between the diaphragm and the capacitor plates. Thus, traditional transducer circuits are not readily adapted for use in miniature transducers, because the errors introduced are often too great to make the transducers of practical value as precision conversion devices. As a result, the output electrical signal, converted from C1-C2, is not linear with respect to the displacement of the diaphragm, and thus accuracy is compromised.
The trend in transducer manufacture, at least with the capacitive displacement type transducer, is reduce the size of the device. This is due primarily to the fabrication of silicon diaphragms according to current silicon processing techniques. Hence, by reducing the size of the silicon diaphragms, many more diaphragms can be fabricated from a silicon wafer, thereby increasing yield. However, smaller diaphragms experience smaller deflections for a given pressure change, and thus smaller capacitance changes which must be sensed by associated electrical circuits. The demands on such circuits require high sensitivity, without compromises in circuit performance due to temperature, parasitic influences or circuit components.
It is also highly desirable to provide a transducer and associated conversion circuit which exhibits a linear relationship between pressure or other input energy, and the output electrical signals. However, standard clamped diaphragm transducers have an inherent nonlinearity between pressure and capacitance. Thus, even with a highly linear circuit, other compensation techniques must be utilized to provide overall linearity. Any transducer or circuit stray capacitance further exacerbates the linearity problem. Costly measures have been undertaken in the circuits to compensate for the nonlinearities. With nonlinearities characterized in the third order, or more, circuit fixes become prohibitively expensive.
It can be seen that a need exists for a transducer system in which a miniature transducer and associated circuit provides an enhanced accuracy and is not sensitive to component tolerances and stray capacitance. Another need exists for a transducer circuit which is cost effective and easily manufactured. Yet another need exists for a technique which corrects for nonlinearities between diaphragm deflection and capacitance change, thereby providing a transducer system with much improved accuracy.